Biosynthetically engineered lipopolysaccharide as vaccine adjuvant

Abstract

Lipopolysaccharide (LPS), a dominant component of the Gram-negative bacterial outer membrane, is a strong activator of the innate immune system, and thereby an important determinant in the adaptive immune response following bacterial infection. This adjuvant activity can be harnessed following immunization with bacteria-derived vaccines that naturally contain LPS, and when LPS or molecules derived from it are added to purified vaccine antigens. However, the downside of the strong biological activity of LPS is its ability to contribute to vaccine reactogenicity. Modification of the LPS structure allows triggering of a proper immune response needed in a vaccine against a particular pathogen while at the same time lowering its toxicity. Extensive modifications to the basic structure are possible by using our current knowledge of bacterial genes involved in LPS biosynthesis and modification. This review focuses on biosynthetic engineering of the structure of LPS and implications of these modifications for generation of safe adjuvants.

Keywords:

• adjuvant
• lipid A
• lipopolysaccharide
• oligosaccharide
• outer membrane vesicle
• pathogenic bacteria
• TLR4
• vaccine

Financial & competing interests disclosure

The authors were supported by Intravacc. P van der Ley is an employee of Intravacc. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Key issues

• Gram-negative bacteria possess lipopolysaccharide (LPS) as a major component of their outer membrane. The lipid A portion of LPS binds to the pattern recognition receptor toll-like receptor 4 (TLR4)/myeloid differentiation protein 2 (MD-2), leading to innate immune signaling. Many pathogenic bacteria can modify their LPS structure to evade this immune recognition and/or resist antimicrobial peptides.

• Biosynthetic engineering of LPS through deletion of LPS biosynthesis genes or induction of modifying enzymes can reduce its endotoxicity while retaining adjuvant activity, resulting in safer live attenuated vaccines, whole cell vaccines and outer membrane vesicle (OMV) vaccines.

• lpxL1 deletion in Neisseria meningitidis has provided a safe OMV vaccine, as shown by a Phase I clinical trial, with reduced endotoxicity while retaining potentially immunogenic outer membrane components. N. meningitidis lgtB mutant oligosaccharide interacts with dendritic cell-specific ICAM-3-grabbing non-integrin, leading to increased uptake and maturation of OMVs by human dendritic cells.

• The combinatorial use of LPS-modifying enzymes in Escherichia coli and other bacteria provides an immense amount of potential LPS structures with different immune-activating properties that can be explored as adjuvants.

• Animal models for testing the adjuvant activity of modified LPS are complicated by the species-specific recognition of different LPS forms by TLR4/MD-2. The construction of a human TLR4/MD-2 transgenic mouse has created a better animal model for testing LPS-based adjuvants. Alternatively, in vitro models using TLR4/MD-2 expressed from different species can be used to predict suitable in vivo models for a specific application.

• Caspase-11 is a newfound pattern recognition receptor for binding cytoplasmic LPS. Deletion of this receptor in mice makes them less susceptible for sepsis after administration of LPS. Its potential role in LPS adjuvant activity needs to be explored.

• Future research will give more insight in different LPS structures and their immune activating potential. New purification methods for preparing homogeneous LPS structures will help determine the full potential of certain modifications made to LPS.